The present invention relates to an apparatus for producing hypochlorite by electrolysis, and in particular to an apparatus easy to handle and used for producing hypochlorite of high concentration.
Hypochlorite, typically represented by sodium hypochlorite, is widely used as bleaching agent or sterilizer in such applications as processing of city water or sewage water, processing of waste water or for use in kitchen at home or for laundry purpose. There are several methods to produce hypochlorite: a method to produce hypochlorite by making chlorine react with alkali metal hydroxide, which is obtained by electrolysis of aqueous solution of alkali metal chloride such as brine, or a method to electrolyze alkali metal chloride in an electrolyzer without diaphragm and to directly produce hypochlorite in the electrolyzer. By the method to make alkali metal hydroxide react with chlorine, hypochlorite of high concentration can be obtained, and this method is adopted in case hypochlorite is produced for the purpose of sale. Because this method requires electrolysis facility to produce alkali metal hydroxide and chlorine, this is carried out in large scale in association with the production of sodium hydroxide or chlorine at factories operated for electrolysis of brine.
FIG. 11 schematically shows a method to produce hypochlorite by reaction of aqueous solution of sodium hydroxide and chlorine, which are obtained by electrolysis of brine. An anode 22 is provided in an anode chamber 21, and a cathode 24 is provided in a cathode chamber 23. In an ion exchange electrolyzer 26 where the anode chamber and the cathode chamber are separated by a cation exchange membrane 25, brine is supplied to the anode chamber, and brine with lower concentration is taken out from the anode chamber 21, and the dissolved chlorine is separated at a dechlorination tower 27. Then, salt is dissolved in a salt adjusting process 28, and brine is purified and is circulated to the anode chamber.
On the other hand, water 29 is supplied to the cathode chamber 23, and while keeping concentration of aqueous solution of sodium hydroxide in cathode solution at a constant level, it is circulated to and from a cathode solution tank 30. Then, the aqueous solution of sodium hydroxide in the cathode solution tank is reacted with chlorine 32, which is generated in the anode chamber of the electrolyzer at an absorption tower 31. Aqueous solution of sodium hypochlorite is produced, and it is stored in a storage tank 33.
Also, a method is known, by which aqueous solution of salt is electrolyzed in an electrolyzer without diaphragm. By this method, concentration of the hypochlorite produced is relatively low, but the product can be directly used for purification of water or sterilization purposes. The manufacturing facility is simple compared with electrolytic facility to produce alkali hydroxide and chlorine, and this product is produced at the site where hypochlorite is needed. Moreover, in the production of hypochlorite by electrolysis, the production yield can be adjusted by changing electric current depending upon the quantity of hypochlorite. In the product thus obtained, chlorine components effective for sterilization are all dissolved in water, and it is easier to use.
Therefore, in the facilities where chlorine or hypochlorite is used, i.e. the facilities where storage equipment for liquid chlorine is installed and gaseous chlorine generated is dissolved or hypochlorite of high concentration is stored and used by diluting, hypochlorite is produced by electrolysis at the site where there is no need to store or to transport the material such as chlorine.
In case of the method to electrolyze aqueous solution of alkali chloride such as common salt using an electrolyzer without diaphragm, the brine supplied as electrolytic solution has concentration of 2% to 4%. The higher the salt concentration is, the higher the efficiency to generate chlorine at the anode is. When the brine containing hypochlorite produced by electrolysis for processing water or other purposes is directly used as the brine with high concentration, the brine with high concentration is intermingled with the water to be processed and this is not desirable. Accordingly, brine having concentration similar to salt concentration of seawater is normally used. In the electrolysis, hypochlorite is produced by reaction of chlorine generated at anode with alkali generated at cathode. If electrolysis is further continued in the electrolyzer, hypochlorite is changed to chlorate. Therefore, even when it is tried to produce hypochlorite of high concentration in the electrolyzer without diaphragm using brine with relatively low concentration as raw material by extending retention time of electrolytic solution, only the quantity of chlorate increases, and production efficiency of hypochlorite is decreased.
In this respect, in order to produce hypochlorite at high electric current efficiency, an electrolyzer has been proposed, in which a plurality of electrolyzers equipped with anodes and cathodes are provided in multiple stages with dividers installed and without increasing electrolytic efficiency in each unit electrolyzer (e.g. JP-B-52-28104 and JP-B-61-44956). However, the concentration of the hypochlorite obtained by such method is not satisfactory for the application purpose, and there have been strong demands on the development of a method, by which hypochlorite of high concentration can be produced by electrolysis with high efficiency. Under such circumstances, the present applicant has proposed a new method in JP-A-5-179475. According to this method, aqueous solution of alkali metal chloride with lower concentration produced at anode chamber of an electrolyzer divided by cation exchange membrane is added to cathode chamber, and alkali aqueous solution containing hypochlorite is obtained, and this is reacted with chlorine produced at the anode chamber, and aqueous solution of hypochlorite can be obtained. By this method, chlorine is reacted with cathode solution. There is regulatory restriction for safety to chlorine as a specific chemical substance and it must be handled in the same manner as in the chlorine producing facility, and it is necessary to take utmost care to prevent leakage of chlorine from piping of the facility.
It is an object of the present invention to provide an apparatus for producing hypochlorite, by which it is possible to produce hypochlorite of high concentration. According to the present invention, it is possible to produce hypochlorite of the same high concentration as in the case where chlorine and sodium hydroxide obtained by electrolysis in ion exchange electrolyzer of brine are used. Further, it is an object of the invention to provide an apparatus for producing hypochlorite with no possibility of chlorine leakage.